Cross-reference is made to co-pending U.S. patent application Ser. No. 7/800,339 to A. Sezginer et al. for "Nuclear Magnetic Resonance Pulse Sequences for Detecting Bound Fluid Volume," filed Nov. 27, 1991.
Nuclear magnetic logging tools, such as disclosed in U.S. Pat. Nos. 4,933,638 to Kenyon et al. for "Borehole Measurement of NMR Characteristics of Earth Formations, and Interpretations Thereof"; and 5,055,787 and 5,055,788 both to Kleinberg et al. for "Borehole Measurement of NMR Characteristics of Earth Formations", measure the number and nuclear magnetic resonance (NMR) relaxation rates of hydrogen atoms in the pore space of rocks by measuring the amplitude and decay rate of signals resulting from pulse-echo sequences. In essence, the nuclear magnetic logging tools send a stream of RF-pulses into the formation and monitor the returning pulses which are called spin echoes. The measurements made are typically cyclical, with each cycle taking up to several seconds. Interpretation algorithms are then used to find the formation properties of interest.
The signal measured by a nuclear magnetic logging tool, such as the Pulsed Nuclear Magnetism Tool (PNMT, mark of Schlumberger) is proportional to the mean density of hydrogen nuclei in the fluid that occupies the pore-space. Hydrogen nuclei in the rock matrix relax too rapidly and are not detected by the tool. Since the hydrogen density in water and liquid hydrocarbons are approximately constant, the detected signal can be calibrated to give the volume fraction of the fluid occupying the pore space.
NMR relaxation of a water saturated porous rock is not a simple exponential relaxation, but it is a continuous superposition of exponential relaxations. For example, in an inversion-recovery (Farrar, T. C. and E. D. Becker, Pulse and Fourier Transform NMR, Academic Press, 1971) experiment, the signal obtained after an inversion and a recovery time of length t is ##EQU1##
Loosely speaking, a(T.sub.1)dT.sub.1 is the volume fraction of the fluid whose relaxation time is between T.sub.1 and T.sub.1 +dT.sub.1, where T.sub.1 is spin-lattice relaxation time. This interpretation is approximately correct because pores of rocks are in a fast diffusion regime (Latour, L. L., R. L. Kleinberg and A. Sezginer, Journal of Coil. and Interf. Science, Vol. 150, No. 2, May 1992) where the NMR signal from each pore is approximately single-exponential, and the relaxation time is proportional to the volume to surface ratio of the pore. Several researchers have demonstrated for water saturated sandstones that the pore size distribution is closely related to the distribution of NMR relaxation times.
Short relaxations times are due to water that is bound to clay minerals or water in pores that are too small to be flushed by a feasible pressure gradient. Also, heavy (viscous) hydrocarbons have shorter relaxation times. Fluids that relax slowly have low viscosity and reside in large pores. Hence, the slowly relaxing fluids can be produced, that is, pumped to the surface, provided there is sufficient permeability. It is therefore important to quantify the volume of the slowly relaxing fluids.
U.S. patent application Ser. No. 7/800,339 describes an NMR pulse sequence for use in a Pulsed Nuclear Magnetic borehole logging tool (PNMT). The pulse sequence includes a series of CPMG pulses according to: EQU T.sub.r -90.degree..sub..+-.x -(t.sub.cp -180.degree..sub.y -t.sub.cp -echo.sub.j)
where j is the index of CPMG echoes gathered. T.sub.r is the wait time, t.sub.cp is the Carr-Purcell spacing. This pulse sequence is used to determine Bound Fluid Volume (BFV) which is subtracted from total porosity to yield Unbound Fluid Volume (UFV) of a formation surrounding the borehole. Measuring the BVF, the amount of rapidly relaxing fluid (that fluid having a spin-spin relaxation time which is less than 33 ms), is more efficient than measuring UFV (up to 2 secs), and is insensitive to motion of the logging tool. However, with this technique, another logging tool is required to determine total porosity. Alternatively, the PNMT itself can be used to determine porosity, however, this is a relatively time consuming technique, which would require two logging passes.